This invention relates to analog to digital converters. In particular, this invention relates to a method and apparatus for high speed, high definition analog to digital conversion of a video signal for use in telecine transfer and other video conversion processes.
Telecine transfer is a process by which a cinematographic film is transferred to video. A telecine apparatus scans a negative or print of a cinematographic film and produces an analog video signal comprising a sequence of values representing the brightness and colour of each image position in the scan. An analog-to-digital converter is used to digitize the analog video signal for storage on a media such as magnetic tape or video disc.
Existing telecine systems represent a trade-off between speed and resolution. When the analog signal is converted to a digital signal, the quantization level of the analog to digital converter (ADC) determines the transfer speed and the resultant resolution of the digital image. For example, an 8 bit ADC digitizes an analog signal at a higher rate than a 12 bit ADC but provides significantly lower resolution.
This balance becomes particularly important in the case of high definition video, which requires a relatively higher quantization resolution in order to accommodate the increased resolution of the image. In prior art analog to digital converters higher resolution can only be achieved at the expense of the transfer rate.
Some prior art systems have combined fine and coarse resolution analog-to-digital converters in an attempt to increase the resolution of the digitized signal without substantially reducing the conversion rate.
For example, U.S. Pat. No. 5,053,771 issued on Oct. 1, 1991 to McDermott, which is incorporated herein by reference, discloses an analog-to-digital converter that utilizes an input signal range selection device to select segments of the input signal for digitization using a finer resolution converter, based on the asymmetric distribution of image information in the analog signal. Setup parameters are stored during a xe2x80x9cpre-scanxe2x80x9d operation which selects those segments of each scene that require higher resolution scanning, and during the transfer process these parameters are used to control the conversion of selected segments of each scene by switching between coarse and fine resolution ADC""s.
This technique effectively pre-scans the film to determine which segments of the scene exposure sequence require a finer quantization to preserve the resolution of the image, and uses this information to control the quantization resolution during actual digitization. However, pre-scanning the film is a time consuming step, and the need to isolate scene segments for fine resolution conversion and control digitization using a prearranged sequence of control signals complicates the converter considerably.
It would accordingly be advantageous to provide an analog to digital converter which provides the high definition of a fine resolution ADC without sacrificing the speed of the conversion or introducing additional steps into the conversion process.
The present invention provides a method and apparatus for converting an analog signal to a digital signal, utilizing an analog to digital converter which provides a high resolution video image suitable for high definition applications at a conversion rate comparable to that of a lower resolution ADC.
The invention accomplishes this by providing an analog to digital converter comprising a diplexer filter which utilizes a fine resolution ADC to digitize the low band component of the analog video signal, and a coarse resolution ADC to digitize the full spectrum of the analog video signal. The coarse resolution signal is subtracted from the fine resolution signal to create a summed signal that includes the difference between the low band components of the respective ADC outputs. The summed signal is filtered through a low pass digital filter to filter out the high band components and pass the quantization error. The coarse resolution signal is delayed as necessary to match any delay in the filter, and the quantization error is added to the output of the coarse resolution ADC to effectively restore the 12 bit resolution of the low band component. The output is a digital signal having a resolution suitable for high definition applications such as HDTV.
The present invention thus provides a method of converting an analog signal to a digital signal, comprising the steps of digitizing a low band component of the analog signal to produce a fine resolution signal, digitizing high and low band components of the analog signal to produce a coarse resolution signal, subtracting the coarse resolution signal from the fine resolution signal to produce a summed signal containing a difference between low band components of the fine resolution signal and the coarse resolution signal, filtering the summed signal through a low pass digital filter to eliminate high band components of the summed signal and pass the difference between low band components, and summing the filtered signal with the coarse resolution signal to produce a digital output signal.
The present invention further provides a converter for converting an analog signal to a digital signal, comprising a fine resolution analog to digital converter for digitizing a low band component of the analog signal and producing a fine resolution digital signal, a coarse resolution analog to digital converter for digitizing high and low band components of the analog signal and producing a coarse resolution digital signal, a processor for subtracting the coarse resolution signal from the fine resolution signal to produce a summed signal containing a difference between low band components of the coarse resolution signal and the fine resolution signal, a low pass digital filter for eliminating a high band component of the summed signal and passing the difference between low band components, and a processor for adding the filtered signal to the coarse resolution signal to produce a digital output signal.
In a preferred embodiment of the invention the fine resolution ADC samples at a lower sampling rate than the coarse resolution ADC and the fine resolution signal is upsampled to the sampling rate of the coarse resolution signal before summing.
In a further preferred embodiment a delay is provided to delay the fine resolution signal to match any delay in the low pass digital filter before summing the quantization error and the coarse resolution signal.